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The Influence of Ambience and Beamline Geometry On Neutron Yield of a Gantry-Mounted Synchrocyclotron Proton Therapy Machine

N Tatari*, M BaradaranGhahfarokhi, T Zhao, A Darafsheh, R Khan, Washington University School of Medicine, St. Louis, MO


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The knowledge of the spatial location of energy dependent neutron fluence can provide important information on safety of the personnel, dose to the patient, shielding design and activation levels in a proton therapy facility. We aim to simulate the neutron production in the various components of the treatment head of a Mevion S250 proton therapy system to determine the contribution of various beamline components and the accelerator ambience towards the neutron yield and spectra.

Methods: The beamline geometry of the treatment head of the Mevion S250 proton therapy system was simulated by Geant4 Monte Carlo code to determine the sources of neutron production. The effects of the room geometry on the neutron spectrum was also studied. A Gaussian shaped monoenergetic proton beam of 250 MeV energy incident on beamline components such as the first scatterer, range modulation wheel (RMW), second scatterer, and the nozzle snout. Neutron yield after the RMW and second scatterer was scored in a plane perpendicular to the beam direction.

Results: Differential angular dependent neutron energy spectrum determined after the RMW and the second scatterer in the proton beamline showed three major peaks – thermal, evaporation, and fast neutrons corresponding to ~0.1 eV, ~1 MeV, and ~100 MeV, respectively. The maximum fraction of in-beam neutrons was found at the location of second scatterer (up to 17%). Compared to the central axis of the beam, the fast neutrons were seen to increase at 15-20o along the beamline (up to 3.5 times).

Conclusion: Second scatterer was one of the major components of neutron production. We have identified certain directions, 15-20o along the proton beam where fast neutron fluence was dominant. Knowledge of preferential directions can allow for more robust component and neutron shielding design of both the head of the proton therapy machines and rooms.

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TH- External Beam- Particle/high LET therapy: Proton therapy – computational dosimetry-Monte Carlo

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